A printing machine is used in the packaging industry for printing flat media such as sheets or a continuous web of paper or cardboard. The machine comprises several stations in succession. A first station situated furthest upstream is an infeed station which inputs the medium in succession. The infeed station supplies several printing stations in the form of one or more printing units placed one after the other. Each printing unit prints a specific color using an ink which has an equivalent coloration. A delivery station which collects the medium which has been printed with an image is provided at the end of the machine.
For printing sheets of cardboard, more particularly corrugated cardboard, the technology used most frequently is flexography using a flexo unit. Digital printing is also developing more and more, using print units equipped with digital print heads, for example of the inkjet-type. Digital printing technology enables the packaging manufacturer to change jobs very quickly in order to print new sheets from a computer file representing the packaging.
The printing machines comprise one or more printing units depending on the number of colors desired. The medium being printed is moved longitudinally from upstream to downstream from the infeed station, to the printing units and as far as the delivery station. In order to obtain a final high-quality image on the printed medium, it is in particular necessary that all the patterns of different colors be superposed exactly. It is also necessary that the printed dots not be deformed.
The packaging is selected mainly according to the criterion of quality level chosen depending on the product which is to be packaged. The medium is currently still quality-controlled visually after printing. The operator looks for all types of faults, closely observing the printed medium, and rejects the medium or media which is or are non-compliant.
The print quality obtained depends not only on the quality of the printing machines, the quality of the inks used, but also the quality of the media input when they enter the machine. Indeed, in the case of cardboard and for the same medium, the surface can have irregularities resulting in different ink absorption properties depending on the nature of the cellulose fibers which make up the medium.
It follows that the printed image applied will have places where there is no ink and other places where there is too much ink, so that the color or colors will vary in intensity, and there will be other faults. When the drop of ink reaches the surface of the medium, it will not spread out uniformly. The image obtained is distorted by a multitude of printed dots, having a much greater surface area than that expected and visible to the naked eye, alternating with dots which appear to be printed correctly. Furthermore, the pigments of the ink are absorbed by the cardboard, resulting in less dense pigmentation density and causing a dull image to be obtained.